As well known from, for example, U.S. Pat. Nos. 3,812,735 and 4,103,566, a key-shift transmission comprises a plurality of speed-change gears rotatably mounted on a transmission shaft, shift keys disposed within elongated axial grooves in the outer surface of the transmission shaft, and a shifter sleeve slidably mounted on the transmission shaft. The shift keys are connected co-slidably to the shifter sleeve which shifts the keys selectively to one of plural positions where the shift keys couple one of the change gears to the transmission shaft.
Control mechanism for shifting the shifter sleeve includes, in general, a latching means for latching the shifter sleeve on the transmission shaft at a neutral position and respective gear-coupling or operative positions. Such latching means prevents an unexpected start of the vehicle due to a spontaneous displacement of the shifter sleeve from the neutral position to an operative position, an unexpected stop of the vehicle due to a spontaneous displacement of the shifter sleeve from an operative position to the neutral position, and an unexpected sudden change in the vehicle speed due to a spontaneous displacement of the shifter sleeve between two operative positions. The latching means also provides, during a shifting operation by an operator, an adequate notice of arrival of the shifter sleeve to a position to be shifted because such latching means suddenly enlarges resisting force against the shifting operation when the shifter sleeve is shifted to an operative or neutral position.
For the purpose of protecting shift keys from damage which may result when the keys engage a rotating change gear having a relatively high speed of rotation, a clutch is usually provided which allows an operator to interrupt the input of power to the transmission so that he or she may performa shifting operation.
In practice, however, a key-shift transmission is often subjected to a shifting operation without disengaging such clutch. That is, shift keys are biased such that gear-engaging clutch portions thereof are projected radially outwardly from elongated axial grooves of a transmission shaft, and thus, even when relative rotation speed between a change gear and transmission shaft is relatively high, the clutch portions can enter clutch-receiving recesses in the inner circumference of the gear at the moment when the clutch-receiving recesses are aligned radially with the clutch portions of shift keys. Consequently, a shifting operation can be performed without disengaging the clutch. Operators are apt to neglect a clutch-disengaging operation at a shifting operation mainly because they dislike a reduction of the vehicle speed.
It is, however, true that, when a shifting operation is performed at a high relative rotational speed between a change gear and transmission shaft, shift keys are subjected to an impact or shock which will result in an earlier decrepitude of the keys.
Accordingly, a primary object of the present invention is to provide an improved key-shift transmission in which an interlocking mechanism for preventing such shifting operation is incorporated in a very simple structure.